The reason for this is found in the Constitution itself. Trump would no longer be incumbent in the Office of the President at the time of the delayed Senate proceeding and would no longer be subject to “impeachment conviction” by the Senate, under the Constitution’s Impeachment Clauses. Which is to say that the Senate’s only power under the Constitution is to convict — or not — an incumbent president.
The purpose, text and structure of the Constitution’s Impeachment Clauses confirm this intuitive and common-sense understanding.
The very concept of constitutional impeachment presupposes the impeachment, conviction and removal of a president who is, at the time of his impeachment, an incumbent in the office from which he is removed. Indeed, that was the purpose of the impeachment power, to remove from office a president or other “civil official” before he could further harm the nation from the office he then occupies.